The invention relates to a device for assisting in the measurement of the earth resistance comprising a ground rod with a wire coiled up on a reel or the like, and electrical connecting means for connecting the wire to the ground rod.
Measurements of the grounding resistance are an essential component of ensuring safety precautions and protective measures when producing, distributing and utilizing electric energy. These protective measures, an essential part of which is the measurement of the grounding resistance, have been laid down in national or international regulations and have to be checked at regular intervals. Measurements of the grounding resistance are, e.g., carried out on the groundings of transformers during operation thereof, high-voltage and medium-voltage towers, tracks, tank farms and boiler plants, the groundings of foundations and also lightning protectors as well as in domestic installations. For this purpose, in most cases two so-called earth rods are driven or rotated into the ground at a certain distance from each other, and they are electrically connected to a measurement instrument by a wire. A further connection will be established between the measurement instrument and the grounded object to be checked, e.g. a utility pole. In this manner, e.g., the grounding resistance of the grounded object is measured. If the value exceeds an upper limit, this is a sign of insufficient grounding.
FR 2 668 138 A1, e.g., describes a means for measuring the earth resistance, comprised of several ground rods electrically interconnected by a wire via a terminal screw. Several reels on which the connecting wires are coiled up are rotatably mounted on a shaft which can be driven by a motor. The reels accommodating the connecting wires and present on the shaft are arranged in a box together with the drive motor, which box may, e.g., be positioned on the loading area of a motor vehicle. This type of arrangement is very large and complex and thus is only suitable for extensive grounding measurements in specific applications.
A ground rod comprising a connecting clamp for the wire is, e.g., known from U.S. Pat. No. 6,137,049, wherein the wire is inserted into the slot of a cap, whereupon this cap is pressed onto the upper end of the ground rod. In this manner, a reliable connection of the wire with the ground rod is achieved.
Apart from such clamping connections, the wires may also be fastened to the ground rod by clamping screws. Such ground rods are described e.g. in WO 98/45895 A1 or in WO 99/27613 A1.
The present invention has as its object to provide a device of the type indicated above, for assisting in the grounding measurement, which is practical to handle and simple, small and which can be produced at low costs. Disadvantages of known arrangements are to be avoided, or reduced, respectively.
This object is achieved according to the invention in that the reel or the like is rotatably mounted on the ground rod. This results in a simple unit which can easily be brought by the respective technical staff to the location of measurement, by rotating or driving the ground rod including the reel into the ground and subsequently uncoiling the connecting wire from the reel. Depending on the length of connecting wire required, larger or smaller reels may rotatably be mounted on the ground rod.
If the ground rod has an offset portion in whose region the reel or the like is rotatable mounted, this will result in an even more compact and space-saving device. Such ground rods provided with a reel may, e.g., be embedded in a tool box or the like provided therefor, and may be transported easily by the respective technical staff to the location of measurement.
Advantageously, the required electrical connecting means between ground rod and wire comprise sliding rings on the rotation axis of the reel or the like. Thus, the connecting wire coiled up on the reel need only be connected with the axis, or with the sliding rings, respectively, whereby automatically an electrical connection of the wire to the ground rod is provided with a rotatable connection of the reel with the ground rod. By providing appropriate electrically conductive lubricants, an increase in the conductivity over the axis can be achieved.
Alternatively, the electrical connecting means between ground rod and wire may comprise plug connections. For this purpose, the bushing may tightly be connected to the ground rod, and the plug may be electrically connected to the connecting wire. Likewise, in a manner known per se, a clamping screw may be provided on the ground rod and connected to the wire end led outside from the interior of the reel.
Advantageously, the reel or the like may be removably mounted on the ground rod. In this manner, the device for measuring the earth resistance may be made smaller, e.g. for the purpose of transportation. Moreover, it is also possible to connect differently sized reels comprising different lengths of wires to one and the same ground rod.
The releasable mounting of the reel on the ground rod advantageously is effected via a snap connection so that the connection can be made quickly and easily.
For an easier coiling of the wire, the reel or the like preferably comprises a crank capable of being removed or folded up. Thereby the wire can be coiled up more easily and more quickly. This removability or pivotability again facilitates transportation of the device, since a more compact unit will be formed.
For longer wires, it may be suitable for the reel or the like to be provided with a drive means, e.g. a coiling spring. By such a measure, coiling up of the wire is substantially facilitated.
The invention will be further explained in more detail by way of the drawings which illustrate an embodiment of the invention.